Protein misfolding caused by exposure to arsenite is associated with transcriptional activation of the AIRAP gene. We report here that AIRAP is an arsenite-inducible subunit of the proteasome's 19S cap that binds near PSMD2 at the 19S base. Compared to the wild-type, knockout mouse cells or C. elegans lacking AIRAP accumulate more polyubiquitylated proteins and exhibit higher levels of stress when exposed to arsenite, and proteasomes isolated from arsenite-treated AIRAP knockout cells are relatively impaired in substrate degradation in vitro. AIRAP's association with the 19S cap reverses the stabilizing affect of ATP on the 26S proteasome during particle purification, and AIRAP-containing proteasomes, though constituted of 19S and 20S subunits, acquire features of hybrid proteasomes with both 19S and 11S regulatory caps. These features include enhanced cleavage of peptide substrates and suggest that AIRAP adapts the cell's core protein degradation machinery to counteract proteotoxicity induced by an environmental toxin.
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We thank Keith Blackwell (Harvard Medical School) for discussing unpublished observations, Michele Pagano and Heather Harding (NYU) and Philip Coffino (UCSF) for insightful comments, Hugh Pelham (MRC Cambridge) for the HDEL antibody, Keiji Tanaka (Tokyo Metropolitan) for the anti-PSMA1-antibody, Rivka Jungreis and David Frescas for assistance with experiments, the BayGenomics consortium for the βGeo insertion ES line, and XM071 and NYU's protein analysis core facility for sample analysis. This work was supported by National Institutes of Health grant ES08681.